Function parseIndexParameter

docs/tests/v2/es6/js/sanddance.js:135389–135395

* Parse parameter for accessing an index of another data set.

(def, spec, scope)

function parseIndexParameter(def, spec, scope) {
  if (!(0, _vegaUtil.isString)(spec.from)) {
    (0, _vegaUtil.error)('Lookup "from" parameter must be a string literal.');
  }

  return scope.getData(spec.from).lookupRef(scope, spec.key);
}
/**
 * Parse a parameter that contains one or more sub-parameter objects.
 */